Electrodes divided by a separator are widely used in many devices that store electrical energy, including primary (non-rechargeable) battery cells, secondary (rechargeable) battery cells, fuel cells, and capacitors. Important characteristics of electrical energy storage devices include energy density, power density during discharge, maximum charging rate, internal leakage current, equivalent series resistance (ESR), and the ability to withstand multiple charge-discharge cycles without unacceptable deterioration in the desirable characteristics of the devices. Electrochemical double layer capacitors, also known as supercapacitors, are gaining popularity in many energy storage applications, because of their high power densities (in both charge and discharge modes), and with energy densities approaching those of conventional rechargeable cells.
Electrical double layer capacitors and HSCs store electrical energy as adsorbed ions in the double layer and/or as redox active compounds deposited onto/into the porous electrode structures, and have very high power density values and very long cycle lives, but moderate energy density values. To increase the energy density, an EDLC-type electrode (with only or predominantly physical adsorption ion processes) can be combined with elements that provide faradic reactions at the electrode surface, resulting in an HSC, in which one electrode's charging/discharging of the electrical double layer takes place, and at another electrode faradic charge transfer processes (redox processes) take place. The faradic reaction capacitance may be very high (sometimes 10 or even more times higher than EDLC capacitance), and thus, the HSC capacitance may substantially determine the energy density. For EDLCs, the columbic efficiency is generally very high (e.g., up to 98%), but for HSCs the columbic efficiency may be somewhat lower (e.g., up to 95%). The development of EDLCs and HSCs is described in Conway (Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications (1999)) and other sources.
The basic structure of a typical EDLC may include a highly porous, sometimes microporous-mesoporous, electrode, charged negatively for cation adsorption; a highly porous, positively charged electrode for anion adsorption; and a polymeric electronically non-conductive and redox inactive porous separator between the negatively and positively charged electrodes. The adsorption/desorption of ions in hybrid/supercapacitors and electrochemical oxidation and/or reduction reactions take place at or inside of the porous structure of the electrodes, to store an electric charge and to generate electric current.
There are three main groups of EDLCs, characterized based on the electrolyte composition used. First, there are non-aqueous EDLCs that use organic solvents. Second, there are aqueous EDLCs. And third, there are EDLCs that use ionic liquids. Usually, in aqueous EDLCs, H2SO4, KOH, and alkali metal salts (Li2SO4, Na2SO4, Rb2SO4, and Mg2SO4) are used. In non-aqueous EDLCs, the selection of salts is broader, including NaPF6, NaClO4, NaBF4, LiBF4, LiPF6, LiClO4, (C2H5)4NBF4, (C2H5)3CH3NBF4, (C2H5)3C3H7ClO4, and others.
In both aqueous and non-aqueous EDLCs and HSCs, the separator may be a porous, electrically non-conductive polymer layer, such as cellulose or polyvinylidene difluoride (PVDF). The operating temperature region may be from −30 to +80 degrees Celsius. A commonly-used EDLC electrode material is a thin layer of porous, amorphous carbon particles, where the amount of binder (such as polytetrafluoroethylene/Teflon/PTFE suspension in water) varies from zero to 30 percent by weight, and the amount of graphite powder (added to increase the electrical conductivity) varies from zero to 30 percent by weight. In some electrode materials, the poly(vinylidene diflouride) solution in dimethylformamide (DMF) and acetone mixture has been used as a binder with zero to 30 percent by weight in dry mass. Various porous carbon particles, like carbon black and amorphous carbons, may be used. The rate of ion adsorption from electrolyte solution, however, may be rather low; one possibility to increase the adsorption/desorption rate is to use hierarchically porous electrodes and/or electrochemically more active redox electrodes (i.e., cathodes and/or anodes). A more attractive possibility is to decrease the adsorption layer thickness, the so-called reaction volume or reaction area, through microporous-mesoporous hierarchically porous structure of the electrode layers. “Micropores” are pores with width lower than 2 nm, while “mesopores” are pores with geometries in the 2 to 50 nanometers range. (This division of pores according to the geometry sizes is given in Short history and types of pores in Silicon, University of Liverpool, Online Master Programmes, available at the time of this writing online at www porous-35.com/porous-si-2.html). Additionally, in the literature “ultramicropores” and “nanopores” (pores with width lower than 0.7 nm) have been discussed, in addition to the three main pore types mentioned above.
To make a conventional electrode, carbon particles, binder, carbon black powder (sometimes added to increase the electrical conductivity of layered electrodes), and one or more solvents are mixed together to form a plastic mass. The mass is passed through rotating drums that apply pressure, resulting in a compact electrode layer. The minimum thickness of the electrode layer achievable depends on the carbon particle size; typically, at least 7 to 10 layers of particles are needed to form an electrode with acceptable mechanical structural characteristics.
Well controlled carbon powders have been prepared using high-temperature chlorination (Cl2) or HCl treatment methods, having porous positive and negative electrodes, each with a hierarchical microporous-mesoporous structure. Preparation of sugar-based carbon nanospheres with very high surface area has been created by three of the inventors herein.
In a typical EDLC using acetonitrile, propylene carbonate, or a mixture of organic carbonates, with specific additives (Sulphur-containing organic compounds, fluorinated carbonates), cell voltages ΔF up to 3.0 V may be applied. For higher ΔE applications, special treatments (such as drying, degassing, reduction) of carbon materials with molecular hydrogen at, for example, 800 degrees Celsius for 2 to 6 hours may be needed.
Generally, EDLCs are manufactured with electrodes divided by a porous separator that prevents the electrodes from coming into contact with each other and thus obstructs electronic current flow directly between the electrodes. The separator is immersed in and impregnated with an electrolyte, and therefore does not stop ionic current flows between the electrodes in both directions. Double layers of charges form at the boundaries between the solid electrodes and the electrolyte.
An electric potential applied between a pair of electrodes of an EDLC causes the electrolyte's ions to be attracted and move towards the electrodes with the opposite charges, generating layers of oppositely-charged ions near each electrode surface. Electrical energy is stored in the charge separation layers between these ionic layers and the charge layers of the corresponding electrode surfaces. In fact, the charge separation layers behave essentially as electrostatic capacitors. Electrostatic energy can also be stored in the double layer capacitors through orientation and alignment of molecules of the electrolytic solution under influence of the electric field induced by the potential.
In comparison to conventional capacitors, double layer capacitors have high capacitance in relation to their volume and weight. There are two main reasons for these volumetric and weight efficiencies. First, the charge separation layers are very narrow. Their widths are typically on the order of nanometers. Second, the electrodes can be made from a porous material, having very large effective surface area per unit volume and per unit weight. Because capacitance is directly proportional to the electrode area and inversely proportional to the widths of the charge separation layers, the combined effects of the large effective surface area and narrow charge separation layers result in capacitance that is very high in comparison to that of conventional capacitors of similar size and weight. High capacitance of double layer capacitors allows the capacitors to receive, store, and release large amounts of electrical energy.
As has already been mentioned, equivalent series resistance is also an important capacitor performance parameter. Frequency response of a capacitor depends on the characteristic time constant of the capacitor, which is essentially a product of the capacitance and the capacitor's equivalent series resistance, or “RC.” To put it differently, equivalent series resistance limits both charge and discharge rates of a capacitor, because the resistance controls the current that flows into or out of the capacitor. Maximizing the charge and discharge rates is important in many applications. In automotive applications, for example, a capacitor used as an energy storage device powering a vehicle's motor has to be able to provide high instantaneous power during acceleration, and to receive bursts of power produced by regenerative braking. In internal combustion vehicles, the energy storage device periodically powers a vehicle's starter, also requiring high power output in relation to the size of the capacitor.
The internal resistance also creates heat during both charge and discharge cycles. Heat causes mechanical stresses and speeds up various chemical reactions, thereby accelerating capacitor aging. Moreover, the energy converted into heat is lost, decreasing the efficiency of the capacitor. It is therefore desirable to reduce equivalent series resistance of capacitors.
Active materials used for electrode construction—activated carbon, for example—may have limited specific conductance. Thus, large contact area may be desired to minimize the interfacial contact resistance between the electrode and its terminal. The active material may also be too brittle or otherwise unsuitable for directly connecting to capacitor terminals. Additionally, the material may have a relatively low tensile strength, needing mechanical support in some applications. For these reasons, electrodes may incorporate current collectors.
A current collector is typically a sheet of conductive material to which the active electrode material is attached. Aluminum foil is commonly used as the current collector of an electrode. In one electrode fabrication process, for example, a film that includes activated carbon powder (i.e., the active electrode material) is produced, and then attached to a thin aluminum foil using an adhesive layer or direct deposition. Thus, for better power density and energy density characteristics, a half-cell or a single cell electrode's sides may be covered by a thin metal film (Al, Ti, Ta, Nb or by Cu or Ni current collectors, for non-aqueous electrolyte and water based systems, respectively) applying magnetron, pulsed laser deposition, physical vapor deposition or other methods, to reduce the series contact resistance values of the dry and filled with electrolyte electrode/membrane half-cells and complete cells.
To improve the quality of the interfacial bond between the film of active electrode material and the current collector, the combination of the film and the current collector may be processed in a pressure laminator. Pressure lamination increases the bonding forces between the film and the current collector, and reduces the equivalent series resistance of the energy storage device in which the electrode is used.
Furthermore, lamination densifies the film of active electrode material, improving the volumetric efficiency of the electrode and of the energy storage device.
It is desirable to provide improved highly ultramicroporous-microporous-mesoporous positively and negatively charged electrodes, separators, half-cells, and complete cells made with two electrodes and a separator. It is desirable to provide energy storage devices including EDLCs, HSCs, Li-ion capacitors and batteries, Na-ion capacitors and batteries, other capacitors and batteries, polymer electrolyte fuel cells, and still other energy storage devices, built with such improved electrodes, separators, half-cells, and complete cells. It is also desirable to provide methods for preparation of such improved electrodes, separators, half-cells, and complete cells, and energy storage devices manufactured with such electrodes, separators, half-cells, and complete cells.